I think Modern for the first film, and then you expand the franchise with the 31st century GOTG. A GotG Trilogy could reveal so much of the Marvel cosmic-verse, alongside the Inhumans and a Nova film.
To be honest,i don't know a thing about GOTG....i have about 4 comics of the old series, (wich had a cameo of doom's brain in a wolverine skeleton...for real!

Is that making any sense?